U.S. Pat. No. 4,221,778 discloses that controlled (i.e., selective, prolonged) continuous release of pharmacologically active drugs, under conditions such as those encountered in the gastrointestinal tract, can be achieved by the application of diffusion barrier coatings to ion exchange resin drug complex particles which have been treated, prior to coating, with an impregnating agent selected from polyethylene glycol, propylene glycol, manitol, lactose, and methylcellulose.